This invention relates to adsorbents for the removal of trace elements, usually present as hydrides, from hydrocarbon streams and a process for their use. More specifically, it relates to a multi-layer adsorbent bed for adsorbing hydrides of arsenic, phosphorus, antimony, boron, silicon, as well as certain sulfur compounds. This invention is particularly useful in the treatment of polymer grade propylene for the manufacture of polypropylene, but also is useful in purification of other hydrocarbon streams.
In addition to the well-known contaminants such as hydrogen sulfide, carbonyl sulfide and mercaptans, light olefin-containing hydrocarbon feedstocks often contain a small quantity of arsine. Usually arsine is present to the extent of only several hundred parts per billion (ppb) by weight. However, even this small amount is normally beyond the allowable limits of an acceptable product (typically less than 20 ppb). The presence of arsine, even at very low concentrations, reduces the polymer yield of olefin catalysts significantly. For example as disclosed in U.S. Pat. No. 4,861,939, at 20° C., 15 bar, WHSV of 6 kg/kg-hr, Ziegler-type catalyst when 305 ppb ArH3 is present, the yield was 10,000 kg polypropylene per kilogram of catalyst, while when there was less than 3 ppb of ArH3, the yield was 32,000 kg polypropylene per kilogram of catalyst. High purity olefins are required for the satisfactory production of many polymeric products, especially those useful as plastics, including polymers of ethylene and propylene. However, arsine is a powerful reducing agent, which appears able to reduce the olefin polymerization catalysts and cause their deactivation. As a result, there has been a real need for improved techniques for removing arsine from light olefin-containing hydrocarbons, especially those used for polymer production.
The purification of propylene and other olefin feed streams is particularly complicated by the small difference between the boiling points of propylene and arsine which hampers arsine removal by fractionation. Consequently, the levels of arsine impurity in propylene stocks are often intolerably high.
The hydrides of boron, silicon, arsenic, phosphorus and antimony are known to be severe catalyst poisons in a variety of processes, including the manufacture of polypropylene and polyethylene. There are other hydrides, including metal hydrides and organometallic hydrides that also act as catalyst poisons. The polymerization reactions to make polypropylene and polyethylene occur over high activity Ziegler-Natta type catalysts or the newly developed metallocene single site catalysts. In order to provide the best catalytic activity of these catalysts, the feed olefin and any other hydrocarbon streams, such as comonomer streams, must be free of contaminants that can bond to the transitional metal groups on the catalyst, thus deactivating the catalyst. The metallocenes are extremely sensitive to arsine and phosphine with sensitivity to levels in the parts per billion (ppb) level range. Most polypropylene manufacturers specify extremely low levels of arsine and phosphine contamination in their propylene supplies with specifications set anywhere from 5 to 50 ppb of either of these impurities. Even the traditional Ziegler-type catalysts, which are less sensitive to these impurities, will produce greatly increased yields upon the removal of the impurities from the propylene. The same issues are present in the manufacture of various other polymers, including polyethylene, polystyrene and various elastomers.
In addition to removal of arsine and phosphines, it is important to remove sulfur compounds. There have been extensive previous efforts to develop adsorbents for the purification of propylene and other hydrocarbons.
U.S. Pat. No. 3,782,076 discloses a process for reducing the arsenic content, believed to be present as arsine, from gaseous hydrocarbon streams by contacting said streams with supported lead oxide. However, the presence of sulfur compounds is said to interfere with the removal of arsine, and furthermore the supported lead oxide may not be regenerated when sulfur compounds are present in the feed.
U.S. Pat. No. 3,833,498 discloses a process for reducing the arsenic content, believed to be present as arsine, from gaseous hydrocarbon streams by contacting said streams with activated carbon derived from a bituminous coal and containing cobalt, nickel, molybdenum and vanadium. However, the feed should be substantially dry and free of sulfur compounds.
U.S. Pat. No. 5,330,560 discloses a process for recovery of arsenic from a gas, such as natural gas, using an inert solid support coated with phosphoric acid and a metal halide, such as ferric chloride or cupric chloride.
U.S. Pat. No. 5,302,771 describes the use of a modified alumina to remove impurities from liquid hydrocarbon streams, such as propylene. The alumina is impregnated with a metal selected from lithium, potassium, calcium, magnesium, barium and sodium.
U.S. Pat. No. 5,990,372 discloses an adsorbent for removal of trace amounts of sulfur, mercury, arsenic, metal hydrides and mixtures thereof, where the adsorbent is a combination of iron oxide, manganese oxide and a support material.
U.S. Pat. No. 6,033,556 discloses the use of a capture mass comprising an alumina support with a metal oxide or sulfide. Metals used included copper, molybdenum, tungsten, iron, nickel and cobalt. The capture masses were used to trap heavy metals, including arsenic, mercury and lead.
U.S. Pat. No. 4,744,221 discloses a method of storing and delivering arsine by contacting a zeolite having a pore size between 5 to 15 angstroms with arsine, at a temperature between −30° C. and 30° C. and then heating the arsine-adsorbed zeolite to a temperature not greater than 175° C. to release a portion of the arsine.
U.S. Pat. No. 5,704,965 discloses a fluid storage and delivery system using a carbon sorbent material that has an affinity for a variety of fluid reagents, including arsine and phosphine.
There still remains a need for improved materials to remove the arsine and phosphine from hydrocarbon streams as well as the sulfur impurities and oxygenates, especially in light of the intolerance of the new polymerization catalysts for these impurities. In particular, there remains a need to overcome the difficulty caused by sulfur compounds being adsorbed and limiting the capacity for the adsorbents to remove arsine and phosphines which, while present in much smaller concentrations than the sulfur compounds, must still be removed.
It is an objective of this invention to provide the simultaneous removal of sulfur compounds, arsine, phosphine and oxygenates, including water, methanol and carbon dioxide from propylene and other hydrocarbon streams.